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I'd say that it's more you can activate Glare's ability by tapping the Sliver, because that's not the Sliver's ability. (The fact it doesn't use the {T} is more relevant if you turn Glare into a creature and use its ability while it still has summoning sickness.)
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ConManOct 16 '13 at 4:38

Wow, I don't think I've seen something so basic asked as a question here. Even if you only START to read the rules of the game, this is covered, and has been in every edition of the rulebook since Revised at the very least.
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Charles BoyungOct 16 '13 at 19:40

3

@CharlesBoyung No need to respond like that. The rules for MTG aren't as straightforward at the beginning. My friends and I took a few weeks before we were confident with what could and could not be used right after a creature was summoned. The fact some things could be used and some couldn't was confusing for a while.
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doppelgreenerOct 17 '13 at 3:51

2 Answers
2

It will have summoning sickness; you cannot active its ability this turn.

From the comprehensive rules:

302.6. A creature's activated ability with the tap symbol or the untap symbol in its activation cost can't be activated unless the creature has been under its controller's control continuously since his or her most recent turn began. A creature can't attack unless it has been under its controller's control continuously since his or her most recent turn began. This rule is informally called the "summoning sickness" rule. Ignore this rule for creatures with haste

A creature that has the Haste ability can tap immediately, so unless you have a Reflex Sliver on the 'field, you will not be able to tap a Gemhide Sliver for mana on the turn it comes out.

Unlike other types of permanents, creatures enter the battlefield with “summoning sickness”: a creature can’t attack, or use an ability that has {T} in its cost, until it has started your turn on the battlefield under your control. You can block with a creature or activate its other abilities no matter how long it’s been on the battlefield.